Image Display Apparatus and Image Display System

ABSTRACT

An image display apparatus having a compact size and appropriately achieving a larger field angle is disclosed while it has two optical systems guiding light from a single image forming device to both eyes. The apparatus has the single image forming device forming an original image, and a first and a second optical system disposed on both sides of a central plane including a central axis of the image forming device. The optical systems guide light from the image forming device to a first and a second eye of an observer, respectively. When light traveling from the image forming device to each pupil of the optical systems is inversely traced from the pupil, each system includes a first surface reflecting the inversely traced light from the pupil in a direction away from the central plane, and a second surface reflecting the inversely traced light from the first surface in a direction away from the central plane.

This application is a continuation of co-pending application Ser. No.10/728,425, filed Dec. 4, 2003, the entire disclosure of which is hereinincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image display apparatus such as ahead mounted display which displays an original image formed in a singleimage forming device to an observer.

2. Description of Related Art

Image display apparatuses of a head mounted type (so-called head mounteddisplays) have conventionally been known in which an image formingdevice such as an LCD (Liquid Crystal Display) is used to enlarginglydisplay an original image displayed in the image forming device throughan optical system.

Since the head mounted display is mounted on the head of an observer,reductions in size and weight of the entire apparatus are particularlyneeded. The display preferably has a small thickness in the optic axisdirection of the observer in consideration of weight balance, appearanceand the like. In addition, it is desirable to display the largestpossible enlarged image to produce a dynamic effect in the image.

For example, each of Japanese Patent Application Laid-Open No. H7(1995)-333551, Japanese Patent Application Laid-Open No. H8(1996)-50256, Japanese Patent Application Laid-Open No. H8(1996)-160340, and Japanese Patent Application Laid-Open No. H8(1996)-179238 has proposed a head mounted display which uses an LCD asan image forming device and a thin prism as an observation opticalsystem to achieve a reduction in thickness of the entire apparatus.

FIG. 16 shows a head mounted display proposed in Japanese PatentApplication Laid-Open No. H7 (1995)-333551. In the head mounted display,light emitted from an LCD 111 is incident on an incident surface 113 ofa small decentered prism 112. The luminous flux is folded by areflective surface 114 and a reflective surface 115 each having acurvature and formed on the prism 112, and then emerges from the surface114 of the decentered prism 112 and is guided to an eye E of anobserver. In this manner, a virtual image of an original image displayedin the LCD 111 is formed and observed by the observer.

The reflective surface 115 of the decentered prism 112 is formed of adecentered free-form surface made of a decentered rotationallyasymmetric surface (a surface exhibiting different optical powerdepending on an azimuth angle, or a so-called free-form surface).

The optical system of the type shown in FIG. 16 is characterized byeasily realizing a reduced thickness of the entire apparatus and alarger field angle of the visual field for observation as compared witha conventional type which uses a coaxial concave mirror and a halfmirror inclined 45 degrees with respect to the optical axis of aneyeball.

In addition, it is desirable that the head mounted display is providedat a low price similarly to other image display apparatuses. As asolution therefor, U.S. Pat. No. 4,322,135 has disclosed a head mounteddisplay of a single original image binocular observation type in which asingle image forming device is used in combination with an opticalsystem which guides a single original image formed in the image formingdevice to each of the left and right eyes of an observer.

FIG. 17 shows the structure of the head mounted display disclosed inU.S. Pat. No. 4,322,135. Image light emitted from a convex originalimage forming surface 202 of an object source 201 for displaying anoriginal image such as an image intensifier is incident on an opticalelement 204 from its transmission area 205, reflected by portions 208each having a reflective film formed thereon or internally reflected bytransmission areas 207 of the optical element 204, reflected by concavemirror portions 203 each having a reflective film formed thereon, andthen emerges from the transmissive areas 207. The light emerging fromthe optical element 204 passes through meniscus lenses 210 and is guidedto both eyes E1 and E2 of an observer.

In addition, various head mounted displays have been proposed in recentyears in which the head mounted display of the single original imagebinocular observation type is used in combination with a prism having adecentered free-form surface as described above. For example, JapanesePatent Application Laid-Open No. H9 (1997)-61748, Japanese PatentApplication Laid-Open No. H9 (1997)-247579, Japanese Patent ApplicationLaid-Open No. 2000-177785, Japanese Patent Application Laid-Open No.2001-194618, and Japanese Patent Application Laid-Open No. 2001-194619have proposed the structure of such displays.

The conventional head mounted displays described above, however, forexample the displays proposed in Japanese Patent Application Laid-OpenNo. H9 (1997)-61748 and Japanese Patent Application Laid-Open No. H9(1997)-247579, involve a high cost since different optical systems areused for both eyes.

In the other examples, the single optical system can be shared betweenboth eyes. However, when the light from the image forming device to thepupil of image light is inversely traced from the pupil side, theinversely traced light is reflected by the first reflective surfaceinward toward the center laterally, so that it is difficult to realize alarger field angle.

The disadvantage is now described with reference to FIG. 18. Lightemerging from an image forming device 311 is incident on an incidentsurface 313L of a small decentered prism 312L. The light is folded by areflective surface 314L and a reflective surface 315L each having acurvature and formed on the prism 312L, and then emerges from thesurface 314L of the decentered prism 312L and is guided to a left eye ELof an observer.

Similarly, light emerging from the image forming device 311 is incidenton an incident surface 313R of a small decentered prism 312R. The lightis folded by a reflective surface 314R and a reflective surface 315Reach having a curvature and formed on the prism 312R, and then emergesfrom the surface 314R of the decentered prism 312R and is guided to aright eye ER of the observer.

In this manner, a virtual image of a single original image displayed inthe image forming device (an LCD) 311 is observed by the left and righteyes EL and ER of the observer.

The following description is made with inversely traced light which isfrequently used in optical design in the virtual image observationsystem. Rays from the eyes of the observer (rays from the pupil) areincident on the prisms 312L and 312R from the surfaces 314L and 314R.The rays are folded by the reflective surfaces 315L and 315R serving asthe first decentered reflective surfaces in this case to approach thenormal to the image forming device 311 displaced substantially at thecenter of both eyes. Thus, in the optical system described above,reflective areas shown by EA1L and EA1R are necessary in the surfaces314L and 314R of the prisms 312L and 312R. These reflective areas EA1Land EA1R become larger as the field angle is larger.

The distance between the left and right eyes EL and ER (interpupillarydistance) of an observer, however, is determined to some extent, so thatit is impossible to extremely increase an interpupillary distance IPDbetween the pupils of the optical systems for left and right eyes.Consequently, a significant increase in field angle cannot be expectedin this structure in which the inversely traced light is folded by thefirst reflective surface inward toward the center laterally.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an image displayapparatus which involves a reduced cost, has a compact size, andappropriately achieves a larger field angle while it has two opticalsystems for guiding light from a single image forming device to botheyes.

To achieve the aforementioned object, according to one aspect of thepresent invention, an image display apparatus comprises a single imageforming device which forms an original image, and a first optical systemand a second optical system which are placed on both sides of a centralplane which includes a central axis of the image forming device, thefirst optical system guiding light from the image forming device to afirst eye of an observer placed near a pupil of the first opticalsystem, and the second optical system guiding light from the imageforming device to a second eye of the observer placed near a pupil ofthe second optical system, wherein, when light traveling from the imageforming device to each of the pupils is inversely traced from the pupil,each of the first and second optical systems includes a first surfacewhich reflects the inversely traced light from the pupil in a directionaway from the central plane, and a second surface which reflects theinversely traced light from the first surface in a direction away fromthe central plane.

According to another aspect of the present invention, an image displayapparatus comprises a single image forming device which forms anoriginal image, and a first optical system and a second optical system,the first optical system guiding light from the image forming device toa first eye of an observer, and the second optical system guiding lightfrom the image forming device to a second eye of the observer, whereineach of the first and second optical systems includes a first surfacewhich reflects light from the image forming device, and a second surfacewhich reflects the light from the first surface back to the firstsurface, and wherein the first surface again reflects the light from thesecond surface.

These and other characteristics of the image display apparatus and imagedisplay system using the same according to the present invention will beapparent from the following description of specific embodiments withreference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the structure of main portions of a head mounted displaywhich is Embodiment 1 of the present invention (showing a centralprincipal ray);

FIG. 2 shows the structure of main portions of the head mounted displaywhich is Embodiment 1 (showing principal rays at the maximum fieldangle);

FIG. 3 shows the structure of main portions of the head mounted displaywhich is Embodiment 1 (showing marginal rays);

FIG. 4 shows the structure of main portions of a head mounted displaywhich is Embodiment 2 of the present invention (showing a centralprincipal ray);

FIG. 5 shows the structure of main portions of the head mounted displaywhich is Embodiment 2 (showing principal rays at the maximum fieldangle);

FIG. 6 shows the structure of main portions of the head mounted displaywhich is Embodiment 2 (showing marginal rays);

FIG. 7 shows the structure of main portions of a head mounted displaywhich is Embodiment 3 of the present invention (showing a centralprincipal ray);

FIG. 8 shows the structure of main portions of the head mounted displaywhich is Embodiment 3 (showing principal rays at the maximum fieldangle);

FIG. 9 shows the structure of main portions of the head mounted displaywhich is Embodiment 3 (showing marginal rays);

FIG. 10 shows the structure of main portions in Numerical Example 1 ofthe present invention;

FIG. 11 shows the structure of main portions in Numerical Example 1;

FIG. 12 shows the structure of main portions in Numerical Example 2 ofthe present invention;

FIG. 13 shows the structure of main portions in Numerical Example 3 ofthe present invention;

FIG. 14 is a plan view showing a variation of Embodiment 3;

FIG. 15 is a perspective view showing a variation of Embodiment 3;

FIG. 16 shows the structure of main portions of a conventional headmounted display;

FIG. 17 shows the structure of main portions of a conventional headmounted display; and

FIG. 18 shows the structure of main portions of a conventional headmounted display.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

FIG. 1 shows the structure of main portions of a head mounted displayserving as an image display apparatus which is Embodiment 1 of thepresent invention. In FIG. 1, reference numeral 1 shows a single imageforming device which forms an original image and is realized by a CRT,an LCD, an electroluminescence device or the like.

A driving circuit 101 is connected to the image forming device 1. Thedriving circuit 101 is supplied with image information from an imageinformation supply apparatus such as a personal computer, a television,a VCR, a DVD player, a reception antenna and a tuner, and drives theimage forming device 1 to display an original image corresponding to theimage information. The image information supply apparatus and the headmounted display constitute an image display system. This applies toEmbodiments 2 and 3, although not shown.

EL shows a left eye (a pupil position) of an observer located at adesirable position, and ER shows a right eye (a pupil position) of theobserver located at a desirable position.

Reference numeral 2 shows a left eye optical system (a first opticalsystem). The left eye optical system 2 has five reflective surfaces froma surface SL1 to a surface SL5 which are disposed to guide light from animage forming surface SI in the image forming device 1 to the left eyeEL.

Reference numeral 3 shows a right eye optical system (a second opticalsystem). The right eye optical system 3 has five reflective surfacesfrom a surface SR1 to a surface SR5 which are disposed to guide lightfrom the image forming surface SI in the image forming device 1 to theright eye ER.

The left eye optical system 2 and the right eye optical system 3 ofEmbodiment 1 are disposed in mirror symmetry laterally with respect to acentral plane perpendicular to the sheet of FIG. 1 and including acentral axis L (shown by a dash dotted line in FIG. 1) which is thenormal to the original image forming surface (which means an effectiveoriginal image forming area, and this applies to the followingdescription) SI of the image forming device 1 passing through the centerthereof. It should be noted that the left and right eye optical systems2 and 3 and the image forming device 1 may not be necessarily placed onthe same plane.

FIG. 2 shows principal rays at the maximum field angle in a horizontalplane (in the sheet of FIG. 2) in Embodiment 1. FIG. 3 shows marginalrays which provide the effective pupil diameter for the center of thefield angle in Embodiment 1.

In FIG. 2, rays emerging from both of the left and right ends in theoriginal image forming surface SI of the image forming device 1 (shownby dotted lines in FIG. 2) are converged to EL and ER which aredesirable pupil positions of the observer to form the exit pupils of theleft eye optical system 2 and the right eye optical system 3.

In FIG. 3, rays emerging from the center of the original image formingsurface SI of the image forming device 1 (shown by solid lines anddotted lines in FIG. 3) are changed into substantially collimated raysby the left eye optical system 2 and the right eye optical system 3 andrecognized by the observer as light from a pixel substantially atinfinity or at a predetermined distance.

Next, optical effects in Embodiment 1 are described with reference toFIGS. 1 to 3. The rays shown by the solid lines in FIGS. 1 to 3 showcentral principal rays (also referred to as central field angleprincipal rays) which emerge from the center of the original imageforming surface SI of the image forming device 1 to the centers of theexit pupils of the left eye and right eye optical systems 2 and 3. Eachreflective surface constituting the optical system of this embodiment isa reflective surface which is decentered with respect to the centralfield angle principal ray.

The light emerging from the center of the original image forming surfaceSI of the image forming device 1 toward the left eye optical system 2 isreflected by the surface SL5 and the surface SL4, and then guided to thesurface SL2. The light reflected by the surface SL2 is reflected by thesurface SL3 back to the surface SL2. The light again reflected by thesurface SL2 is reflected by the surface SL1 and guided to the left eyeEL of the observer.

Similarly, the light emerging from the center of the original imageforming surface SI of the image forming device 1 toward the right eyeoptical system 3 is reflected by the surface SR5 and the surface SR4,and then guided to the surface SR2. The light reflected by the surfaceSR2 is reflected by the surface SR3 back to the surface SR2. The lightagain reflected by the surface SR2 is reflected by the surface SR1 andguided to the right eye ER of the observer.

In this event, in the left eye optical system 2, the light reflected bythe surface SL4 travels in a direction toward the left and away from thecentral axis L between the optical systems 2 and 3 shown by the dashdotted line in FIGS. 1 to 3 and is incident on the surface SL2, andfurther reflected thereby in a direction toward the left and away fromthe central axis L. The light is reflected by the surface SL3 back tothe surface SL2 in the opposite direction, that is, in a directionapproaching the central axis L. The light is then reflected again by thesurface SL2 and travels in a direction approaching the central axis L.

In this manner, the surface SL2 is used as a surface by which the lightis reflected twice, and the surface SL3 is provided to serve as areturning reflective surface for reflecting the light back in theopposite direction between the two reflections. Thus, a go and returnoptical path is formed such that the light travels from the surfacesSL2, SL3, and then to SL2. This reduces the size of the optical systemby the overlapping optical paths in the optical system. Particularly,the returning reflective surface SL3 reflects the light such that thetraveling directions of the light after the two respective reflectionsby the surface SL2 are opposite to each other laterally to make theoptical system compact.

The light reflected by the surface SL3 in the direction approaching thecentral axis L and further reflected by the surface SL2 in the directionapproaching the central axis L is reflected by the surface SL1 towardthe left eye EL of the observer. In other words, when the rays travelingfrom the image forming device 1 toward the pupil are inversely tracedfrom the pupil, the inversely traced rays are bent by the surface SL1 inan outward direction away from the central axis L (that is, the centralplane), and further bent by the surface SL2 in the outward directionaway from the central axis L. This can reduce the number of opticalelements disposed between the rays toward the left eye EL and thecentral axis L, and advantage can be taken of that reduction to increasethe field angle, thereby making it possible to employ an opticalarrangement suitable for a larger field angle.

The left eye optical system 2 has the structure in which a plurality ofdecentered reflective surfaces are used to fold the optical path. Thiscan be combined with the go and return optical path provided asdescribed above to confine a large optical path length within the smalloptical system.

It is thus possible to use the structure of one-time image formation inwhich an intermediate image of the original image formed on the originalimage forming surface SI of the image forming device 1 is once formed inthe left eye optical system 2 and the intermediate image is enlarged andpresented as a virtual image.

In Embodiment 1, for example as shown in FIG. 3, the intermediate image(a real image) is formed on an intermediate image forming surface IMafter the reflection by the surface SL3 and before the re-reflection bythe surface SL2. The surfaces SL5, SL4, SL2, and SL3 are used as a relayoptical system, while the surfaces SL2 and SL1 are used as an ocularoptical system. This can increase the flexibility in field angle settingfor the size of the original image forming surface SI of the imageforming device 1 to achieve a larger field angle.

In this event, to reduce the number of the optical elements in the relayoptical system and the ocular optical system, the intermediate image maybe curved as appropriate to have an astigmatic difference such thataberration caused by the relay optical system and aberration caused bythe ocular optical system cancel out each other. This structure canprevent an increase in the number of the optical elements while opticalperformance of the entire system is maintained.

In Embodiment 1, at least two of the decentered reflective surfaces SL1,SL2, SL3, SL4, and SL5 constituting the left eye optical system 2 areformed as curved surfaces to enlargingly present an image of theoriginal image formed on the original image forming surface SI of theimage forming device 1. If the number of curved reflective surfaces isincreased among the decentered reflective surfaces, it is possible toreduce the number of optical elements which do not contribute to imageformation or aberration correction. As a result, a cost reduction andimprovement in optical performance can be achieved.

More desirably, all the decentered reflective surfaces may be formed ascurved surfaces to further reduce the cost and improve opticalperformance.

The surface SL3 reflects the central principal ray back substantially inthe opposite direction to provide the aforementioned go and returnoptical path. The surface SL3 is preferably formed to satisfy thefollowing expression:θ<45°where θ represents an angle (in the absolute value) formed between thecentral principal ray as the incident ray on the surface SL3 after thereflection by the surface SL2 and the central principal ray as theemerging (reflected) ray traveling toward the surface SL2 after thereflection by the surface SL3 as shown in FIG. 1.

If the angle θ is equal to or larger than 45°, it is difficult for thesurface SL3 to function as the returning reflective surface and thus toform the go and return optical path. Consequently, the size of theoptical system is extremely increased.

More desirably, the following is satisfied:θ<30°If the angle θ is equal to or larger than 30°, the surface SL2 has alarge effective surface size to cause difficulty in making the entireoptical system compact, although the surface SL3 can have the functionas the returning reflective surface.

When the decentered reflective surface is formed as a curved surface,rotationally asymmetric aberration or so-called decentering aberrationoccurs which is not caused in a conventional coaxial and rotationallysymmetric optical system. It is thus preferable that at least one of thedecentered reflective surfaces is formed as a rotationally asymmetricshape to correct the decentering aberration. Since an increased numberof rotationally asymmetric surfaces enhances the flexibility incorrection of the decentering aberration, a plurality of surfaces aredesirably formed as rotationally asymmetric surfaces.

More desirably, all the decentered reflective curved surfaces may beformed in a rotationally asymmetric shape to achieve significantlyfavorable optical performance.

The above description for the structure and the optical effects of theleft eye optical system 2 applies to the right eye optical system 3.Thus, the head mounted display as a whole can have an optical structurewhich involves a reduced cost, has a compact size, and appropriatelyachieves a larger field angle while guiding an image of the originalimage formed in the single image forming device 1 to the left and righteyes.

It is desirable that the aforementioned rotationally asymmetric surfaceis plane-symmetric longitudinally (in a direction perpendicular to thesheet of the figures) with respect to a horizontal plane (the sheet)passing through the center of the original image forming surface SI ofthe image forming device 1 and the center between the exit pupils of theleft and right eye optical systems 2 and 3 (in other words, the centralaxis L). Since the structure described above can provide a referenceplane for manufacturing and assembly, the manufacturing and assembly arefacilitated to produce the effect of a cost reduction. In addition, withsuch a surface shape, the left eye optical system 2 and the right eyeoptical system 3 have the same elements disposed at positions rotated180 degrees about the central axis L. The left eye optical system 2 andthe right eye optical system 3 can be realized with the same elements,so that the effect of a manufacturing cost reduction can be produced.

Embodiment 2

FIG. 4 shows the structure of main portions of a head mounted displaywhich is Embodiment 2 of the present invention.

In FIG. 4, reference numeral 11 shows a single image forming devicewhich forms an original image. EL shows a left eye (a pupil position) ofan observer located at a desirable position, and ER shows a right eye (apupil position) of the observer located at a desirable position.

Reference numeral 12 shows a left eye optical system (a first opticalsystem). The left eye optical system 12 has a prism 21 which has threesurfaces of a surface SL1, a surface SL2 having a semi-transmissivereflective film formed thereon, and a surface SL3 having a reflectivefilm formed thereon. The optical system 12 also has a lens 22 which hastransmissive surfaces SL4 and SL5. These are arranged to guide lightfrom an original image forming surface SI of the image forming device 11to the left eye EL.

Reference numeral 13 shows a right eye optical system. The right eyeoptical system 13 has a prism 31 which has three surfaces of a surfaceSR1, a surface SR2 having a semi-transmissive reflective film formedthereon, and a surface SR3 having a reflective film formed thereon. Theoptical system 13 also has a lens 32 which has transmissive surfaces SR4and SR5. These are arranged to guide light from the original imageforming surface SI of the image forming device 11 to the right eye ER.

The left eye optical system 12 and the right eye optical system 13 ofEmbodiment 2 are disposed in mirror symmetry laterally with respect to acentral plane perpendicular to the sheet of FIG. 4 and including acentral axis L (shown by a dash dotted line in FIG. 4) which is thenormal to the original image forming surface SI of the image formingdevice 11 passing through the center thereof.

Such a structure can realize the left eye optical system 12 and theright eye optical system 13 with the same elements, so that the effectof a manufacturing cost reduction can be provided.

FIG. 5 shows principal rays at the maximum field angle in a horizontalplane (in the sheet) in Embodiment 2. FIG. 6 shows marginal rays whichprovide the effective pupil diameter for the center of the field anglein Embodiment 2.

In FIG. 5, rays emerging from both of the left and right ends in theoriginal image forming surface SI of the image forming device 11 areconverged to EL and EL which are desirable pupil positions of theobserver to form exit pupils of the left eye optical system 12 and theright eye optical system 13.

In FIG. 6, rays emerging from the center of the original image formingsurface SI of the image forming device 11 (shown by solid lines anddotted lines in FIG. 6) are changed into substantially collimated raysby the left eye optical system 12 and the right eye optical system 13and recognized by the observer as light from a pixel substantially atinfinity or at a predetermined longer distance as compared with thedistance from the exit pupil position to the original image formingsurface SI.

Next, optical effects in Embodiment 2 are described with reference toFIGS. 4 to 6. The rays shown by solid lines in FIGS. 4 to 6 show centralprincipal rays which emerge from the center of the original imageforming surface SI of the image forming device 11 to the centers of thepupils of the left eye and right eye optical systems 12 and 13.

The light emerging from the center of the original image forming surfaceSI of the image forming device 11 toward the left eye optical system 12is refracted and transmitted by the surface SL5 and the surface SL4 ofthe lens 22, and then guided to the surface SL2 of the prism 21. Thelight refracted and transmitted by the surface SL2 and entering theprism 21 is reflected by the surface SL1 and then reflected by thesurface SL3 back to the surface SL1.

The light reflected again by the surface SL1 is reflected by the surfaceSL2 and then travels again toward the surface SL1. The light toward thesurface SL1 is now transmitted by the surface SL1 and guided to the lefteye EL of the observer.

Similarly, the light emerging from the center of the original imageforming surface SI of the image forming device 11 toward the right eyeoptical system 13 is refracted and transmitted by the surface SR5 andthe surface SR4 of the lens 32, and then guided to the surface SR2 ofthe prism 31. The light refracted and transmitted by the surface SR2 andentering the prism 31 is reflected by the surface SR1 and then reflectedby the surface SR3 back to the surface SR1.

The light reflected again by the surface SR1 is reflected by the surfaceSR2 and then travels again toward the surface SR1. The light toward thesurface SR1 is now transmitted by the surface SR1 and guided to theright eye ER of the observer.

The reflection and re-reflection by the surfaces SL1 and SR1 arerealized by semi-transmissive reflective films (half mirrors) formed onthe surfaces SL1 and SR1 or by internal total reflection (the totalreflection is occurred by incidence of rays at a larger incident anglethan a critical angle) in the prisms 21 and 31. The use of the internaltotal reflection is preferable since the light use efficiency can beincreased to display a bright image.

It is also possible that no reflective film is provided but internaltotal reflection is used in each area of the surfaces SL1 and SR1through which effective rays pass and a reflective film is formed toreflect rays in the other portions. In this case, the flexibility ofoptical design can be increased without significantly reducingbrightness to improve the optical performance and reduce the size of theoptical system as compared with the case where all the rays aresubjected to internal total reflection.

When the reflective film is formed on part of each of the surfaces SL1and SR1 in this manner to provide both of the reflective area using thereflective film and the reflective area using the internal totalreflection in the single surface, a gradation reflective film isdesirably used to have a gradually increasing thickness toward thereflective area using the reflective film from the reflective area usingthe internal total reflection (in a direction away from the central axisL shown by the dash dotted line in this case) near the boundary betweenthe areas. This is preferable since the boundary between the reflectivearea using the reflective film and the reflective area using theinternal total reflection is not conspicuous.

In Embodiment 2, the surfaces SL1 and SR1 are used to produce both thedecentering reflection effect twice and the transmission effect toreduce the number of necessary optical elements. In addition, thesurface SL2 and SR2 are used to produce both the transmission effect andthe decentering reflection effect to further reduce the number ofoptical elements.

In the left eye optical system 12, the light incident on the prism 21from the surface SL2 travels in a direction (to the left) away from thecentral axis L between the left and right eye optical systems 12 and 13shown by the dash dotted line in FIGS. 4 to 6, and is incident on thesurface SL1. The light is further reflected in a direction away from thecentral axis L and incident on the surface SL3. Then, the light isreflected by the surface SL3 back to the surface SL1 in the oppositedirection, that is, in a direction approaching the central axis L, andis reflected again by the surface SL1 and travels in a directionapproaching the central axis L.

In this manner, the surface SL1 is used as a surface by which the lightis reflected twice, and the surface SL3 is provided to serve as areturning reflective surface for reflecting the light back in theopposite direction between the two reflections. Thus, a go and returnoptical path is formed such that the light travels from the surfacesSL1, SL3, and then to SL1. This reduces the size of the optical systemby the overlapping optical paths in the optical system. Particularly,the returning reflective surface SL3 reflects the light such that thetraveling directions of the light in the two reflections by the surfaceSL1 are opposite to each other laterally to make the optical systemcompact.

In Embodiment 2, by adding the surface SL2 which allows the light toenter the prism 21 and serves as the final reflective concave mirror inthe prism 21, a go and return optical path is formed such that the lighttravels from the surfaces SL2, SL1, SL3, SL1, and then to SL2 to providethe overlapping optical paths. This increases the overlap of the lightto extremely reduce the size of the optical system for the optical pathlength.

In Embodiment 2, the light reflected by the surface SL1 in the directionapproaching the central axis L is guided by the surface SL2 toward theleft eye EL of the observer. In other words, when the rays travelingfrom the image forming device 11 toward the pupil are inversely tracedfrom the pupil, the inversely traced rays are bent by the surface SL2 inan outward direction away from the central axis L (that is, the centralplane), and further bent by the surface SL1 in the outward directionaway from the central axis L. This can reduce the size of the area whichis optically used between the rays toward the left eye EL and thecentral axis L for the field angle, and advantage can be taken of thatreduction to increase the field angle, thereby making it possible toemploy an optical arrangement suitable for a larger field angle.

Embodiment 2 employs the structure in which a plurality of decenteredreflective surfaces SL1 and SL2 are used to fold the optical path toachieve a reduced thickness. The structure can be combined with the goand return optical path provided as described above to ensure a largeoptical path length in the small and thin optical system. It is thuspossible to employ the structure of one-time image formation in which anintermediate image of the original image formed in the original imageforming surface SI of the image forming device 11 is once formed in theleft eye optical system 12 and the intermediate image is enlarged andpresented as a virtual image while the optical system is formed in acompact size.

In Embodiment 2, for example as shown in FIG. 6, the intermediate image(a real image) is formed on an intermediate image forming position IMafter the reflection by the surface SL3 and before the re-reflection bythe surface SL1. The surfaces SL5, SL4, SL2, SL1, and SL3 are used as arelay optical system, while the surfaces SL1, SL2, and SL1 are used asan ocular optical system. This can increase the flexibility in fieldangle setting for the size of the original image forming surface SI ofthe image forming device 11 to achieve a larger field angle.

In this event, to reduce the number of the optical elements in the relayoptical system and the ocular optical system, the intermediate image maybe curved as appropriate to have an astigmatic difference such thataberration caused by the relay optical system and aberration caused bythe ocular optical system cancel out each other. This structure canprevent an increase in the number of the optical elements while opticalperformance of the entire system is maintained.

In the structure of Embodiment 2, at least one of the two decenteredreflective surfaces SL1 and SL2 constituting part of the left eyeoptical system 12 is formed as a curved surface to enlargingly presentthe original image formed in the original image forming surface SI ofthe image forming device 11. If the number of curved reflective surfaceis increased among the decentered reflective surfaces, it is possible toreduce the number of optical elements which do not contribute to imageformation or aberration correction. As a result, a cost reduction andimprovement in optical performance can be achieved. That is, both of thetwo decentered reflective surfaces may be formed as curved surfaces toallow a cost reduction and improvement in optical performance.

Since the SL3 is disposed at an angle to reflect the central principalray substantially perpendicularly in order to provide the aforementionedgo and return optical path, the surface SL3 is not necessarilydecentered with respect to the central principal ray.

The surface SL3 is preferably formed to satisfy the followingexpression:θ<45°where θ represents an angle (in the absolute value) formed between thecentral principal ray as the incident ray on the surface SL3 after thereflection by the surface SL1 and the central principal ray as theemerging (reflected) ray traveling toward the surface SL1 after thereflection by the surface SL3 as shown in FIG. 4.

If the angle θ is equal to or larger than 45°, it is difficult for thesurface SL3 to function as the returning reflective surface and thus toform the go and return optical path. Consequently, the optical systemstructure of Embodiment 2 is not possible.

More desirably, the following is satisfied:θ<30°If the angle θ is equal to or larger than 30°, the surface SL1 has alarge effective surface size to cause difficulty in making the entireoptical system compact, although the surface SL3 can have the functionas the returning reflective surface.

Thus, it is desirable that the surface SL3 is not decentered withrespect to the central principal ray (θ=0°) or is a decenteredreflective surface in the aforementioned range of the condition for θ.Similarly to the aforementioned decentered reflective surface, when thesurface SL3 serving as the returning reflective surface is formed as acurved surface, the number of optical elements which do not contributeto image formation or aberration correction can be reduced to achieve acost reduction and improvement in optical performance.

When the decentered reflective surface is formed as a curved surface,rotationally asymmetric aberration or so-called decentering aberrationoccurs which is not caused in a conventional coaxial and rotationallysymmetric optical system. Thus, it is preferable that at least one ofthe decentered reflective surfaces is formed in a rotationallyasymmetric shape to correct the decentering aberration.

Since an increased number of rotationally asymmetric surfaces enhancesthe flexibility in correction of the decentering aberration, a pluralityof surfaces are desirably formed as rotationally asymmetric surfaces.More desirably, all the decentered reflective curved surfaces may beformed in a rotationally asymmetric shape to achieve significantlyfavorable optical performance. When the surface SL3 is formed in arotationally asymmetric shape, the flexibility in correction of thedecentering aberration can be more increased to realize favorableoptical performance.

The above description for the structure and the optical effects of theleft eye optical system 12 applies to the right eye optical system 13.Thus, the head mounted display as a whole has an optical structure whichinvolves a reduced cost, has a compact size, and appropriately achievesa larger field angle while guiding an image of the original image formedin the single image forming device 11 to the left and right eyes.

It is desirable that the aforementioned rotationally asymmetric surfacesare plane-symmetric longitudinally (in a direction perpendicular to thesheet of the figures) with respect to a horizontal plane (the sheet)passing through the center of the original image forming surface SI ofthe image forming device 11 and the center between the exit pupils ofthe left and right eye optical systems 12 and 13 (in other words, thecentral axis L). Since the structure described above can provide areference plane for manufacturing and assembly, the manufacturing andassembly are facilitated to provide the effect of a cost reduction.Particularly, when the horizontal plane is the only symmetry plane, theflexibility in optical design can be increased to achieve favorableoptical performance while the reference plane for manufacturing andassembly is maintained. In addition, with such a surface shape, the lefteye optical system 12 and the right eye optical system 13 have the sameelements arranged at the positions rotated 180 degrees about the centralaxis L. The left eye optical system 12 and the right eye optical system13 can be achieved with the common parts, so that the effect of amanufacturing cost reduction can be provided.

Embodiment 3

FIG. 7 shows the structure of main portions of a head mounted displaywhich is Embodiment 3 of the present invention. In FIG. 7, referencenumeral 41 shows a single image forming device which forms an originalimage. EL shows a left eye (a pupil position) of an observer located ata desirable position, and ER shows a right eye (a pupil position) of theobserver located at a desirable position.

Reference numeral 42 shows a left eye optical system(a first opticalsystem). The left eye optical system 42 has a prism 53 which has threesurfaces of a surface SL1 having a reflective film formed on at least aportion thereof, a surface SL2 having a semi-transmissive reflectivefilm formed thereon, and a surface SL3 having a reflective film formedthereon. The optical system 42 also has a prism 54 which has thecemented surface SL2 to the prism 53, a surface SL4 having a reflectivefilm formed thereon, and a surface SL5 as a transmissive surface. Theseare arranged to guide light from an original image forming surface SI ofthe image forming device 41 to the left eye EL.

Reference numeral 43 shows a right eye optical system. The right eyeoptical system 43 has a prism 63 which has three surfaces of a surfaceSR1 having a reflective film formed on at least a portion thereof, asurface SR2 having a semi-transmissive reflective film formed thereon,and a surface SR3 having a reflective film formed thereon. The opticalsystem 43 also has a prism 64 which has the cemented surface SR2 to theprism 63, a surface SL4 having a reflective film formed thereon, and asurface SL5 as a transmissive surface. These are arranged to guide lightfrom the original image forming surface SI of the image forming device41 to the right eye ER.

The left eye optical system 42 and the right eye optical system 43 ofEmbodiment 3 are disposed in mirror symmetry laterally with respect to acentral plane perpendicular to the sheet of FIG. 7 and including acentral axis L (shown by a dash dotted line in FIG. 7) which is thenormal to the original image forming surface SI of the image formingdevice 41 passing through the center thereof.

Although details are later described, it is desirable in Embodiment 3that respective optical surfaces constituting the left eye opticalsystem 42 and the right eye optical system 43 are symmetrically shapedperpendicularly with respect to the sheet (a horizontal plane includingthe central axis L). Thus, the left eye optical system 42 and the righteye optical system 43 have the same elements arranged at the positionsrotated 180 degrees about the central axis L (shown by the dash dottedline in FIG. 7). This structure allows the left eye optical system 42and the right eye optical system 43 to be realized with the sameelements, so that the effect of a manufacturing cost reduction can beprovided.

FIG. 8 shows principal rays at the maximum field angle in a horizontalplane (in the sheet) in Embodiment 3. FIG. 9 shows marginal rays whichform the effective pupil diameter for the center of the field angle inEmbodiment 3.

In FIG. 8, rays emerging from both of the left and right ends of theoriginal image forming surface SI (effective display area) of the imageforming device 41 are converged to the left eye EL and the right eye ELwhich are desirable pupil positions of the observer to form the exitpupils of the left eye optical system 42 and the right eye opticalsystem 43.

In FIG. 9, rays emerging from the center of the original image formingsurface SI of the image forming device 41 are changed into substantiallycollimated rays by the left eye optical system 42 and the right eyeoptical system 43 and recognized by the observer as light from a pixelat infinity or at a predetermined longer distance as compared with thedistance from the exit pupil position to the original image formingsurface SI.

Next, optical effects in Embodiment 3 are described with reference toFIGS. 7 to 9. The rays shown by solid lines in FIGS. 7 to 9 show centralprincipal rays which emerge from the center of the original imageforming surface SI of the image forming device 41 to the centers of thepupils of the left eye and right eye optical systems 42 and 43.

The light emerging from the center of the original image forming surfaceSI of the image forming device 41 toward the left eye optical system 42is refracted and transmitted by the surface SL5 and enters the prism 54,reflected by the surface SL4, and guided to the surface SL2. The surfaceSL2 is the cemented surface between the prism 54 and the prism 53, andis a half mirror surface having the semi-transmissive reflective filmformed on at least one side thereof.

Part of the light guided to the surface SL2 is transmitted by thesurface SL2 and enters the prism 53. The light entering the prism 53 isreflected by the surface SL1 and then by the surface SL3, again travelstoward the surface SL1, and is reflected by the surface SL1 back to thesurface SL3. The light again reflected by the surface SL3 travels towardthe area of the surface SL1 where the light is first reflected, and isreflected by the surface SL1.

Part of the light reflected by the surface SL1 is reflected by thesurface SL2 which is the half mirror surface, and then travels towardthe surface SL1. The light is now transmitted by the surface SL1 andguided to the left eye EL of the observer.

Similarly, the light emerging from the center of the original imageforming surface SI of the image forming device 41 toward the right eyeoptical system 43 is refracted and transmitted by the surface SR5 andenters the prism 64, reflected by the surface SR4, and guided to thesurface SR2. The surface SR2 is the cemented surface between the prism64 and the prism 63, and is a half mirror surface having thesemi-transmissive reflective film formed on at least one side thereof.

Part of the light guided to the surface SR2 is transmitted by thesurface SR2 and enters the prism 63. The light entering the prism 63 isreflected by the surface SR1 and then by the surface SR3, again travelstoward the surface SR1, and is reflected by the surface SR1 back to thesurface SR3. The light again reflected by the surface SR3 travels towardthe area of the surface SR1 where the light is first reflected, and thenreflected by the surface SR1. Part of the light reflected by the surfaceSR1 is reflected by the surface SR2 which is the half mirror surface,and then travels toward the surface SR1. The light is now transmitted bythe surface SR1 and guided to the right eye ER of the observer.

The first reflection and the third reflection by the surfaces SL1 andSR1 are realized by half mirrors having semi-transmissive reflectivefilms formed on the surfaces SL1 and SR1 or by internal total reflectionin the prisms 53 and 63. The use of the internal total reflection canincrease the light use efficiency to display a bright image.

It is also possible that no reflective film is provided but internaltotal reflection is used in each area of the surfaces SL1 and SR1through which effective rays pass and a reflective film is formed toreflect light in the other portions. In this case, the flexibility ofoptical design can be increased without significantly reducingbrightness to improve optical performance and reduce the size of theoptical system as compared with the case where all the rays aresubjected to the internal total reflection.

The second reflection by the surfaces SL1 and SR1 is returningreflection in which the surfaces SL1 and SR1 receive the light after thefirst reflection by the surfaces SL1 and SR1 serving as the decenteredreflective surface through the surfaces SL3 and SR3 serving as thedecentered reflective surfaces to reflect the light back toward thesurfaces SL3 and SR3 to result in the third reflection realized by thesurfaces SL1 and SR1 in the opposite direction laterally to thetraveling direction of the light in the first reflection. Thus, it isnecessary to form a reflective film in each effective reflection areafor the second reflection of the surfaces SL1 and SR1.

When the reflective film is formed on part of each of the surfaces SL1and SR1 in this manner to provide both the reflective area using thereflective film and the reflective area using the internal totalreflection or the semi-transmissive reflective area in the singlesurface, a gradation reflective film is desirably used to have agradually increasing thickness toward the reflective area using thereflective film from the reflective area using the internal totalreflection or the semi-transmissive reflective area (in a direction awayfrom the central axis L in this case) near the boundary between theareas. This is preferable since the boundary between the reflectiveareas is not conspicuous.

In Embodiment 3, the surfaces SL1 and SR1 are used to produce thedecentering reflection effect twice, the returning reflection effect,and the transmission effect to reduce the number of necessary opticalelements. In addition, the surface SL2 and SR2 are used to produce boththe transmission effect and the decentering reflection effect to furtherreduce the number of optical elements.

In the left eye optical system 42, the light incident on the prism 53from the surface SL2 travels in a direction (to the left) away from thecentral axis L shown by the dash dotted line in FIGS. 7 to 9, and isincident on the surface SL1. The light is further reflected by thesurface SL1 in a direction away from the central axis L, reflected anddeflected by the surface SL3, and travels in a direction incident on thesurface SL1 substantially perpendicularly. The surface SL1 reflects thelight in the opposite direction back to the surface SL3. The surface SL3reflects the light in a direction approaching the central axis L, andthe surface SL1 reflects the light in a direction further approachingthe central axis L.

In this manner, a go and return optical path is formed such that thelight travels from the surfaces SL1, SL3, SL1, SL3, and then to SL1.This reduces the size of the optical system by the overlapping opticalpaths in the optical system.

In Embodiment 3, by adding the surface SL2 which allows the light toenter the prism 53 and serves as the final reflective concave surface inthe prism 53, a go and return optical path is formed such that the lighttravels from the surfaces SL2, SL1, SL3, SL1, SL3, SL1, and then to SL2to provide the overlapping optical paths. This increases the overlap ofthe light to extremely reduce the size of the optical system for theoptical path length.

In Embodiment 3, the light reflected by the surface SL1 in the directionapproaching the central axis L is guided by the surface SL2 toward theleft eye EL of the observer. In other words, when the rays travelingfrom the image forming device 41 toward the pupil are inversely tracedfrom the pupil, the inversely traced rays are bent by the surface SL2 inan outward direction away from the central axis L (that is, the centralplane), and further bent by the surface SL1 in the outward directionaway from the central axis L.

This can reduce the size of the area which is optically used between therays toward the left eye EL and the central axis L for the field angle,and advantage can be taken of that reduction to increase the fieldangle, thereby making it possible to employ an optical arrangementsuitable for a larger field angle.

Embodiment 3 employs the structure in which the plurality of decenteredreflective surfaces SL1, SL2, and SL3 are used to fold the optical pathto achieve a reduced thickness. The structure can be combined with thego and return optical path provided as described above to ensure a largeoptical path length in the small and thin optical system. It is thuspossible to employ the structure of one-time image formation in which anintermediate image of the original image formed on the original imageforming surface SI of the image forming device 41 is once formed in theleft eye optical system 42 and the intermediate image is enlarged andpresented as a virtual image while the optical system is formed in acompact size.

In Embodiment 3, for example as shown in FIG. 9, the intermediate image(a real image) is formed on an intermediate image forming position IMafter the reflection by the surface SL3 and before the reflection by thesurface SL1 (the returning reflection). The surfaces SL5, SL4, SL2, SL1,SL3, and SL1 are used as a relay optical system, while the surfaces SL3,SL1, SL2, and SL1 are used as an ocular optical system. This canincrease the flexibility in field angle setting for the size of theoriginal image forming surface SI of the image forming device 41 toachieve a larger field angle.

In this event, to reduce the number of the optical elements in the relayoptical system and the ocular optical system, the intermediate image maybe curved as appropriate to have an astigmatic difference such thataberration caused by the relay optical system and aberration caused bythe ocular optical system cancel out each other. This structure canprevent an increase in the number of the optical elements while opticalperformance of the entire system is maintained.

In the structure of Embodiment 3, at least one of the three decenteredreflective surfaces SL1, SL2, and SL3 constituting part of the left eyeoptical system 42 is formed as a curved surface to enlargingly presentan image of the original image formed on the original image formingsurface SI of the image forming device 41.

If the number of curved reflective surfaces is increased among thedecentered reflective surfaces, it is possible to reduce the number ofoptical elements which do not contribute to image formation oraberration correction. As a result, a cost reduction and improvement inoptical performance can be achieved. That is, all the three decenteredreflective surfaces may be formed as curved surfaces to allow a costreduction and improvement in optical performance.

The surface SL1 is preferably formed to satisfy the followingexpression:θ<45°where θ represents an angle (in the absolute value) formed between thecentral principal ray as the incident ray on the surface SL1 and thecentral principal ray as the emerging (reflected) ray therefrom in thesecond reflection in which the central principal ray is reflected on thesurface SL1 toward the surface SL3 after the first reflection by thesurface SL1 and the first reflection by the surface SL3 as shown in FIG.7.

If the angle θ is equal to or larger than 45°, it is difficult for thesurface SL1 to function as the returning reflective surface and thus toform the go and return optical path.

More desirably, the following is satisfied:θ<30°

If the angle θ is equal to or larger than 30°, the surface SL3 and thesurface SL1 have a large effective surface size to cause difficulty inmaking the entire optical system compact, although the surface SL1 canhave the function as the returning reflective surface.

Thus, it is desirable that the surface SL1 in the second reflection isnot decentered with respect to the central principal ray (θ=0°) or is adecentered reflective surface in the aforementioned range of thecondition for θ.

When the decentered reflective surface is formed as a curved surface,rotationally asymmetric aberration or so-called decentering aberrationoccurs which is not caused in a conventional coaxial and rotationallysymmetric optical system. Thus, it is preferable that at least one ofthe decentered reflective surfaces is formed in a rotationallyasymmetric shape to correct the decentering aberration. Since anincreased number of rotationally asymmetric surfaces enhances theflexibility in correction of the decentering aberration, a plurality ofsurfaces are desirably formed as rotationally asymmetric surfaces.

More desirably, all the decentered reflective curved surfaces may beformed in a rotationally asymmetric shape to achieve significantlyfavorable optical performance.

The above description for the structure and the optical effects of theleft eye optical system 42 applies to the right eye optical system 43.Thus, the head mounted display as a whole has an optical structure whichinvolves a reduced cost, has a compact size, and appropriately achievesa larger field angle while guiding an image of the original image formedin the single image forming device 41 to the left and right eyes in theoptical system.

It is desirable that the aforementioned rotationally asymmetric surfacesare plane-symmetric longitudinally (in a direction perpendicular to thesheet of the figures) with respect to a horizontal plane (the sheet)passing through the center of the original image forming surface SI ofthe image forming device 41 and the center between the exit pupils (inother words, the central axis L) of the left and right eye opticalsystems 42 and 43. Since the structure described above can provide areference plane for manufacturing and assembly, the manufacturing andassembly are facilitated to provide the effect of a cost reduction.Particularly, when the horizontal plane is the only symmetry plane, theflexibility in optical design can be increased to achieve favorableoptical performance while the reference plane for manufacturing andassembly is maintained.

When the image forming device 41 is disposed to protrude to some extentfrom the optical systems 42 and 43 toward the observer, the optical pathmay be bent at a position B shown by a dotted line in FIG. 14 off thehorizontal plane (in the sheet of the figure) including the centralprincipal ray.

FIG. 15 is a perspective view showing the left eye optical system whenthe optical path is bent perpendicularly to the sheet of FIG. 14 byadditionally providing a mirror SL6. The bent optical path by the mirrorSL6 in this manner can easily provide space for avoiding the nose of theobserver required when the head mounted display is constituted. Thiseliminates the need to reduce a back focal distance with enormousefforts to easily achieve high optical performance.

In each of Embodiments 1 to 3 described above, the reflective filmrefers to a film by which substantially no light is transmitted. A filmby which part of light is transmitted is referred to as asemi-transmissive reflective film for distinction from the reflectivefilm. However, the semi-transmissive reflective film is not limited to afilm by which light is transmitted and reflected at the same ratios.

NUMERICAL EXAMPLE 1

FIGS. 10 and 11 are schematic diagrams for explaining Numerical Example1 of the present invention. Numerical Example 1 is associated withEmbodiment 3 described above.

In FIG. 10, SI shows the original image forming surface of the imageforming device 41. SL1, SL2, SL3, SL4, and SL5 show the optical surfacesconstituting the left eye optical system 42. SLS shows the exit pupil ofthe left eye optical system 42.

SR1, SR2, SR3, SR4, and SR5 show the optical surfaces constituting theright eye optical system 43. SRS shows the exit pupil of the right eyeoptical system 43.

As described above as the preferable structure in Embodiment 3, each ofthe surfaces in the left eye optical system 42 and the right eye opticalsystem 43 is formed in plane symmetry which has a yz section which isthe plane of the sheet of the figure as the only plane of symmetry, sothat the left and right optical systems have the same structures. Thus,the following description is made for only one of the optical systems(the left eye optical system 42). This applies to Numerical Examples 2and 3 below.

In the following description, a coordinate system described below isused. The center of the pupil SLS is defined as the origin point. A zaxis is defined as the direction of the optic axis (the directionmatches the central principal ray between the pupil SLS and the planeSL1). A y axis is defined as the direction perpendicular to the z axison the sheet (a plane including the central principal ray). An x axis isdefined such that the direction penetrating the sheet is positive toform a right-handed system. The surfaces arranged in the coordinatesystem are designated with the surface numbers in order in which therays from the original image forming surface SL1 to the pupil SLS areinversely traced.

Thus, as shown in FIG. 11, the pupil is shown as S1. The surface SL1 inFIG. 10 is shown as an incident surface S2 of the inversely traced rays,a decentered reflective surface S4, a returning reflective surface S6,and a decentered reflective surface S8. The surface SL2 is shown as adecentered reflective surface S3 and a transmissive surface S9. Thesurface SL3 is shown as decentered reflective surfaces S5 and S7. Thesurface SL4 is shown as a reflective surface S10. The surface SL5 isshown as a transmissive surface S11. The original image forming surfaceis shown as SI without change.

The surface arrangement is decentered only in the yx section, and thusrotation is caused about the x axis. This is represented by A (indegrees (°)) with a counterclockwise direction defined as positive.

Optical data tables show the surface number SURF, the positions X, Y, Zof each surface, and rotational angle A of each surface. The radius ofcurvature is represented by R, the type of surface definition by typ,the refractive index and the Abbe number of a medium after the surfaceby nd and vd. In addition, e−M represents 10^(−M).

Air is represented by Nd=1 and vd=0, and the sign of the value of Nd ischanged when the surface reflects light. For the type of surfacedefinition, SPH represents a spherical surface expressed only by thevalue of the radius of curvature R, and SPS represents a rotationallyasymmetric surface which is represented by the following expression (1)and has coefficients shown in lower portions of Table 1 corresponding tothe number given after SPS. When not shown in Table 1, the coefficientin the term is equal to zero. $\begin{matrix}{{z = {\frac{{cr}^{2}}{1 + {{SQRT}\lbrack {1 - {( {1 + k} )c^{2}r^{2}}} \rbrack}} + {\sum\limits_{i = 2}^{66}{c_{j}x^{m}y^{n}}}}}{j = {{\lbrack {( {m + n} )^{2} + m + {3n}} \rbrack/2} + 1}}{{{{where}\quad c} = {1/R}},\quad{r = \sqrt{( {x^{2} + y^{2}} )}}}} & (1)\end{matrix}$

-   -   The above description applies to Numerical Examples 2 to 3.

Table 1 shows data in Numerical Example 1. The data can be used torealize a optical system with a horizontal (y direction) field angle of30°, a vertical field angle of approximately 23°, and a pupil diameterof 10 mm for an original image forming surface of approximately 0.5inches diagonally (10.2 mm×7.6 mm) when the length is represented inmillimeters.

In terms of optical design, rays (inversely traced rays) coming from apoint at infinity and passing through the pupil S1 are incident on thefirst prism from the surface S2, reflected by the surfaces S3, S4, S5,S6, S7, and S8, incident on the second prism from the surface S9,reflected by the surface S10, emerge from the surface S11, and thenformed into an image on the original image forming surface SI.

Thus, rays from the original image forming surface SI are guided to thepupil S1 in the reverse path to the aforementioned one, and an observerhaving his or her pupil at the position of the pupil S1 can recognize anenlarged virtual image of a horizontal field angle of 30° at infinity.TABLE 1 SURF X Y Z A R typ Nd νd 1 0.000 0.000 0.000 0.000 ∞ SPH 1.00000.0 2 0.000 −7.285 26.153 1.115 −305.2224 SPS1 1.5709 33.8 3 0.000−6.526 36.298 −25.878 −96.8602 SPS2 — 33.8 4 0.000 −7.285 26.153 1.115−305.2224 SPS1 1.5709 33.8 5 0.000 14.164 47.382 24.189 −99.7291 SPS3 —33.8 6 0.000 −7.285 26.153 1.115 −305.2224 SPS1 1.5709 33.8 7 0.00014.164 47.382 24.189 −99.7291 SPS3 — 33.8 8 0.000 −7.285 26.153 1.115−305.2224 SPS1 1.5709 33.8 9 0.000 −6.526 36.298 −25.878 −96.8602 SPS21.5709 33.8 10  0.000 −5.956 48.165 0.485 −90.7507 SPS4 — 33.8 11  0.000−16.855 31.486 28.440 8.4304 SPS5 — 0.0 I 0.000 −31.500 20.819 0.000 ∞SPH — 0.0 SPS1 k: 1.8613e+01 c 4: −6.6076e−03 c 6: 4.0901e− c 8:−15400e− c10: −6.1349e− c11: 2.4803e−06 c13: −5.9119e− c15: −1.7999e−SPS2 k: −1.8861e+00 c 4: −1.3956e−03 c 6: −1.4504e− c 8: −1.2584e− c10:−1.4678e− c11: 4.9186e−07 c13: 1.9753e− c15: 9.2345e−07 SPS3 k:2.8152e+00 c 4: −5.0991e−03 c 6: 1.2520e− c 8: −2.5645e− c10: 2.0148e−c11: 9.0811e−07 c13: −5.5939e− c15: −2.8068e− SPS4 k: −3.2053e+01 c 4:−5.6389e−03 c 6: 1.5074e− c 8: 1.5579e− c10: 1.0478e− c11: 1.6326e−05c13: 3.3390e− c15: −2.8989e− SPS5 k: −8.5159e−01 c 4: −1.2880e−02 c 6:−7.9671e− c 8: 3.5790e− c10: 2.8434e− c11: 2.9525e−05 c13: 5.4985e− c15:−3.2370e−

NUMERICAL EXAMPLE 2

FIG. 12 shows the structure of main portions of an optical system inNumerical Example 2 of the present invention. Numerical Example 2represents a variation of Embodiment 2 described above. A first prism P1has the same form as the prisms 21 and 31 described in Embodiment 2, anda second prism P2 is used instead of the lenses 22 and 32. Table 2 showsoptical data of Numerical Example 2.

Symbols and the like in Table 2 are basically identical to those inTable 1. However, ZRN* given as typ means a rotationally asymmetricalsurface which is represented by the following expression (2) and hascoefficients shown in lower portions of Table 2. When not shown in Table2, the coefficient in the term is equal to zero. $\begin{matrix}{z = {{( {1/R} ){( {x^{2} + y^{2}} )/( {1 + ( {1 - {( {1 + k} )( {1/R} )^{2}( {x^{2} + y^{2}} )}} )^{({1/2})}} )}} + {c\quad 2} + {c\quad 4} + {c\quad 5( {x^{2} - y^{2}} )} + {c\quad 6( {{- 1} + {2x^{2}} + {2y^{2}}} )} + {c\quad 10( {{{- 1}y} + {3x^{2}y} + {3y^{3}}} )} + {c\quad 11( {{3x^{2}y} - y^{3}} )} + {c\quad 12\quad( {x^{4} - {6x^{2}y^{2}} + y^{4}} )} + {c\quad 13( {{{- 3}x^{2}} + {4x^{4}} + {3y^{2}} - {4y^{4}}} )} + {c\quad 14( {1 - {6x^{2}} + {6x^{4}} - {6y^{2}} + {12x^{2}y^{2}} + {6y^{4}}} )} + {c\quad 20( {{3y} - {12x^{2}y} + {10x^{4}y} - {12y^{3}} + {20x^{2}y^{3}} + {10y^{5}}} )} + {c\quad 21( {{{- 12}x^{2}y} + {15x^{4}y} + {4y^{3}} + {10x^{2}y^{3}} - {5y^{5}}} )} + {c\quad 22( {{5x^{4}y} - {10x^{2}y^{3}} + y^{5}} )} + {c\quad 23( {x^{6} - {15x^{4}y^{2}} + {15x^{2}y^{4}} - y^{6}} )} + {c\quad 24( {{{- 5}x^{4}} + {6x^{6}} + {30x^{2}y^{2}} - {30x^{4}y^{2}} - {5y^{4}} - {30x^{2}y^{4}} + {6y^{6}}} )} + {c\quad 25( {{6x^{2}} - {20x^{4}} + {15x^{6}} - {6y^{2}} + {15x^{4}y^{2}} + {20y^{4}} - {15z^{2}y^{4}} - {15y^{6}}} )} + {c\quad 26( {{- 1} + {12x^{2}} - {30x^{4}} + {20x^{6}} + {12y^{2}} - {60x^{2}y^{2}} + {60x^{4}y^{2}} - {30y^{4}} + {60x^{2}y^{4}} + {20y^{6}}} )} + \ldots}} & (2)\end{matrix}$

The data of Numerical Example 2 can be used to realize a optical systemwith a horizontal (y direction) field angle of 30°, a vertical fieldangle of approximately 23°, and a pupil diameter of 10 mm for anoriginal image forming surface of approximately 0.5 inches diagonally(10.2 mm×7.6 mm) when the length is represented in millimeters.

In terms of optical design, rays(inversely traced rays) coming from apoint at infinity and passing through the pupil S1 are incident on thefirst prism P1 from the surface S2, reflected by the surfaces S3, S4,S5, and S6, and emerge from the surface S7. The rays are incident on thesecond prism from the surface S8, reflected by the surface S9, emergefrom the surface S10, and then formed into an image on the originalimage forming surface SI. Thus, rays from the original image formingsurface SI are guided to the pupil S1 in the reverse path to theaforementioned one, and an observer having his or her pupil at theposition of the pupil S1 can recognize an enlarged virtual image of ahorizontal field angle of 30° at infinity. TABLE 2 SURF X Y Z A R typ Nνd 1 0.000 0.000 0.000 0.000 ∞ SPH 1.0000 0.0 2 0.000 0.158 22.135 0.935−969.1174 ZRN1 1.5709 33. 3 0.000 −0.164 33.984 −24.920 −79.9320 ZRN2−1.5709 33. 4 0.000 0.158 22.135 0.935 −969.1174 ZRN1 1.5709 33. 5 0.00029.681 37.582 51.267 5285.1651 ZRN3 −1.5709 33. 6 0.000 0.158 22.1350.935 −969.1174 ZRN1 1.5709 33. 7 0.000 −0.164 33.984 −24.920 −79.9320ZRN2 1.0000 0.0 8 0.000 −1.539 38.661 −40.635 41.9482 ZRN4 1.5709 33. 90.000 −9.331 49.816 −6.412 −92.7568 ZRN5 −1.5709 33. 10  0.000 −20.35334.311 35.754 94.4157 ZRN6 −1.0000 0.0 I 0.000 −31.500 22.640 0.000 ∞SPH −1.0000 0.0 ZRN1 k: −5.7790e+02 c 5: — c 6: 3.0603e−05 c10: 7.6267e−c11: −3.2089e−06 c12: c13: −5.3606e−09 c14: 6.2601e− ZRN2 k: −1.2151e−01c 5: — c 6: −6.9358e−05 c10: 2.2529e− c11: −1.0524e−05 c12: — c13:−1.4494e−07 c14: −2.0724e− ZRN3 k: 2.8728e−08 c 5: 2.2643e− c 6:4.7085e−05 c10: 1.0668e− c11: 6.2695e−05 c12: c13: 3.7011e−07 c14:6.2614e− ZRN4 k: −9.1681e+00 c 5: 4.9406e− c 6: 3.3844e−03 c10:−1.4247e− c11: 8.8604e−05 c12: — c13: −1.4664e−06 c14: −2.4981e− ZRN5 k:6.7671e−01 c 5: — c 6: −2.9929e−04 c10: 4.1222e− c11: 8.3064e−06 c12:c13: 7.5899e−09 c14: −4.2980e− ZRN6 k: 4.2956e+01 c 5: 6.3112e− c 6:3.5215e−03 c10: 1.3921e− c11: −1.0260e−04 c12: c13: −1.1530e−06 c14:−6.3036e−

NUMERICAL EXAMPLE 3

FIG. 13 shows the structure of main portions of an optical system inNumerical Example 3 of the present invention. Numerical Example 3represents a variation of Embodiment 2 described above. A first prism P1has the same form as the prisms 21 and 31 described in Embodiment 2, anda second prism P2 is used instead of the lenses 22 and 32.

Numerical Example 3 differs from Numerical Example 2 in that the numberof reflections by the second prism P2 is increased. Table 3 showsoptical data of Numerical Example 3.

Symbols and the like in Table 3 are basically identical to those inTable 1. However, ZRN* given as typ means a rotationally asymmetricalsurface which is represented by the aforementioned expression (2) andhas coefficients shown in lower portions of Table 3. When not shown inTable 3, the coefficient in the term is equal to zero.

The data of Numerical Example 3 can be used to realize a optical systemwith a horizontal (y direction) field angle of 30°, a vertical fieldangle of approximately 23°, and a pupil diameter of 10 mm for anoriginal image forming surface of approximately 0.5 inches diagonally(10.2 mm×7.6 mm) when the length is represented in millimeters.

In terms of optical design, rays(inversely traced rays) coming from apoint at infinity and passing through the pupil S1 are incident on thefirst prism P1 from the surface S2, reflected by the surfaces S3, S4,S5, and S6, and emerge from the surface S7. The rays are incident on thesecond prism from the surface S8, reflected by the surfaces S9 and S10,emerge from the surface S11, and then formed into an image on thedisplay surface SI.

Thus, rays from the display surface SI are guided to the pupil SI in thereverse path to the aforementioned one, and an observer having his orher pupil at the position of the pupil S1 can recognize an enlargedvirtual image of a horizontal field angle of 30° at infinity. TABLE 3SURF X Y Z A R typ Nd νd 1 0.000 0.000 0.000 0.000 ∞ SPH 1.0000 0.0 20.000 −0.444 22.104 0.077 −875.0947 ZRN1 1.5709 33.8 3 0.000 −1.32037.620 — −78.0960 ZRN2 — 33.8 4 0.000 −0.444 22.104 0.077 −875.0947 ZRN11.5709 33.8 5 0.000 26.725 34.349 50.908 1452.5933 ZRN3 — 33.8 6 0.000−0.444 22.104 0.077 −875.0947 ZRN1 1.5709 33.8 7 0.000 −1.320 37.620 —−78.0960 ZRN2 1.0000 0.0 8 0.000 −3.386 42.254 — 40.4687 ZRN4 1.570933.8 9 0.000 — 51.441 — −234.2743 ZRN5 — 33.8 10  0.000 — 32.621 −8.71693.9123 ZRN6 1.5709 33.8 11  0.000 — 51.441 — −234.2743 ZRN5 1.0000 0.0I 0.000 — 59.749 0.000 ∞ SPH 1.0000 0.0 ZRN1 k: 5.6705e+02 c 5:−4.0120e−05 c 6: 1.5454e− c10: −2.7041e− c11: −7.6781e−06 c12:2.3706e−07 c13: −3.0584e− c14: −6.4053e− ZRN2 k: 7.8049e−01 c 5:−6.9760e−04 c 6: −1.2034e− c10: 3.2462e− c11: −4.7675e−06 c12:7.2306e−09 c13: −5.9395e− c14: −2.4312e− ZRN3 k: −1.6665e−05 c 5:1.5589e−03 c 6: 1.7149e− c10: 4.9476e− c11: −8.8798e−05 c12: 6.7369e−07c13: 3.0032e− c14: 3.2767e− ZRN4 k: −4.3323e+00 c 5: 7.2856e−04 c 6:5.5245e− c10: 1.62486− c11: 8.7530e−06 c12: 1.8919e−06 c13: −3.0799e−c14: −1.4376e− ZRN5 k: 2.1189e+01 c 5: 7.3049e−06 c 6: −1.1164e− c10:3.0150e− c11: 3.0374e−06 c12: 7.8107e−08 c13: 7.4630e− c14: −2.8119e−ZRN6 k: −5.6498e+00 c 5: 2.6091e−04 c 6: 1.2751e− c10: 5.21296− c11:−1.3827e−06 c12: −9.0770e−08 c13: 1.9531e− c14: −9.4946e−

As described above, according to Embodiments 1 to 3, a larger fieldangle can be provided while the distance between the exit pupils of theleft and right optical systems is maintained approximately at thedistance between eyes of a usual observer.

The so-called go and return optical paths formed in the left and rightoptical systems can reduce the size of the optical systems to providethe compact image display apparatus.

In each of Embodiments 1 to 3, the left eye optical system and the righteye optical system can be formed in mirror symmetry laterally withrespect to the central plane which passes through the center of theimage display device and in plane symmetry longitudinally with respectto the horizontal plane to use the common optical elements in the leftand right optical system to reduce the cost.

The total number of the optical elements can be reduced by forming atleast one of the plurality of reflective surfaces including the firstand second surfaces as a decentered reflective surface. In addition,various types of aberration can be favorably corrected to improveoptical performance by forming at least one reflective surface as arotationally asymmetric surface.

Furthermore, in the left eye optical system and the right eye opticalsystem, the intermediate real image of the original image is formed toallow display of an image at a large field angle even when the originalimage (that is, the image display device) has a small size.

While preferred embodiments have been described, it is to be understoodthat modification and variation of the present invention may be madewithout departing from the scope of the following claims.

1. An image display apparatus comprising: a single image forming devicewhich forms an original image; and a first optical system and a secondoptical system which are disposed on both sides of a central plane whichincludes a central axis of the image forming device, the first opticalsystem guiding light from the image forming device to a first eye of anobserver placed near a pupil of the first optical system, and the secondoptical system guiding light from the image forming device to a secondeye of the observer placed near a pupil of the second optical system,wherein, when light traveling from the image forming device to each ofthe pupils is inversely traced from the pupil, each of the first andsecond optical systems includes: a first surface which reflects theinversely traced light from the pupil in a direction away from thecentral plane; and a second surface which reflects the inversely tracedlight from the first surface in a direction away from the central plane.2. The image display apparatus according to claim 1, wherein the firstand second optical systems are arranged in mirror symmetry with respectto the central plane.
 3. An image display apparatus comprising: a singleimage forming device which forms an original image; and a first opticalsystem and a second optical system, the first optical system guidinglight from the image forming device to a first eye of an observer, andthe second optical system guiding light from the image forming device toa second eye of the observer, wherein each of the first and secondoptical systems includes: a first surface which reflects light from theimage forming device; and a second surface which reflects the light fromthe first surface back to the first surface, wherein the first surfaceagain reflects the light from the second surface.
 4. The image displayapparatus according to claim 3, wherein the first optical system and thesecond optical system are disposed on both sides of a central planewhich includes a central axis of the image forming device, and the firstoptical system and the second optical system are arranged in mirrorsymmetry with respect to the central plane.
 5. The image displayapparatus according to claim 3, wherein the first optical system and thesecond optical system are disposed on both sides of a central planewhich includes a central axis of the image forming device, and each ofthe first optical system and the second optical system is arranged inplane symmetry with respect to a plane perpendicular to the centralplane.
 6. The image display apparatus according to claim 3, wherein eachof the first and second optical systems includes a plurality ofreflective surfaces including the first and second surfaces, and atleast one of the plurality of reflective surfaces is a decentered curvedsurface.
 7. The image display apparatus according to claim 3, whereineach of the first and second optical systems includes a plurality ofreflective surfaces including the first and second surfaces, and atleast one of the plurality of reflective surfaces is a rotationallyasymmetric surface.
 8. The image display apparatus according to claim 3,wherein intermediate image is formed from light from the image formingdevice within each of the first and second optical systems.
 9. The imagedisplay apparatus according to claim 3, wherein each of the first andsecond optical systems includes an optical element which has the firstand second reflective surfaces formed integrally on a transparent body,and at least one optical surface of the optical element serves as aninternal total reflection surface and a transmissive surface dependingon an incident angle of light.
 10. The image display apparatus accordingto claim 3, wherein the following expression is satisfied when a raytraveling from the center of an original image forming area of the imageforming device to the center of a pupil of each of the first and secondoptical systems is defined as a central principal ray:θ<45° where θ represents an angle formed between the central principalray as an incident ray on the second surface and the central principalray as a reflected ray from the second surface when the centralprincipal ray reflected by the first surface is incident on the secondsurface, reflected by the second surface, and then travels back towardthe first surface.
 11. An image display system comprising: the imagedisplay apparatus according to claim 1; and an image information supplyapparatus which supplies image information for enabling the imageforming device to form an original image to the image display apparatus.12. An image display system comprising: the image display apparatusaccording to claim 3; and an image information supply apparatus whichsupplies image information for enabling the image forming device to forman original image to the image display apparatus.